As the need for energy constantly increases while fossil fuel reserves continue to dwindle, renewable energy sources are rapidly gaining importance. Renewable energy can be obtained using solar, geothermal, or wind-turbine technology. Although significant research has been made into each of those areas, prior art designs have typically not yielded the compact, economical, and efficient generators needed for, among other things, renewable energy sources (e.g., wind turbines).
Rotating electrical machines—both generators and motors—are devices that transform mechanical power into electrical power, and vice-versa. A rotating electrical machine generally includes a magnetic field and an armature that rotate with respect to each other. In doing so, the armature is the part of the machine in which the energy conversion takes place.
In rotating direct current (DC) machines, the magnetic field (provided by the “stator”) often is static and the armature rotates within it (the rotor). In rotating alternating current (AC) machines, the armature typically is stationary and the magnetic field rotates with respect to the armature. The armature generally has windings, which are conductors that are move with respect to the magnetic field in order to produce a voltage. The armature winding is usually placed on the stator.
DC machines once were widely employed in the industry and commonly were easily controlled by supply voltage adjustment. At this point in time, AC motors were not widely employed because they typically require frequency changes that are usually difficult to accomplish. With advances in power electronics and microprocessors, however, precise frequency control became more easily achievable in AC motors. As a result, AC motors generally have replaced or superseded DC motors, which are relatively expensive and usually require frequent maintenance. AC electricity also is easier than DC to distribute widely and utilize in a wide range of applications.
In AC machines generally, electromotive force (EMF) is generated when magnetic flux linking armature winding changes in time. The amount of EMF is proportional to the number of turns of the winding and the time derivative of the flux. The resulting torque is a result of forces produced in individual conductors of the armature winding when the winding is linked by magnetic flux and carries the armature current. For the torque to maintain one direction, the armature current must alternate with the same frequency as that of the magnetic field.
AC motors, for example, are typically built in a single or three-phase arrangement of the winding in which EMF is generated. In such AC machines, current also flows in the armature winding, thereby producing the electromagnetic torque. The associated voltage and current are typically referred to as armature voltage and armature current respectively. In a typical AC motor, EMF opposes the armature current and the torque causes the motion of the rotor.
AC generators, for example, typically have a permanent magnet that rotates inside a coil in such a way that the N-pole and S-pole alternate as seen from the coil. The speed of rotation determines the number of cycles per second, or frequency, usually measured in Hertz(Hz). A rotation speed of 3000 revolutions per minute (RPM), for example, produces 50 Hz, and 3600 RPM produces 60 Hz. In a typical AC generator, EMF causes the flow of armature current while torque opposes the motion of the rotor.
Certain AC generators and motors, called synchronous machines, are commonly used as generators for large power systems, such as turbine generators and hydroelectric generators in a grid power supply. A synchronous machine generally refers to a machine whose speed is directly proportional to the frequency of voltage produced in it, or supplied to it. In the former case, the machine operates as an AC generator, and in the latter as an AC motor. The generated armature EMF and voltage and, in presence of an electrical load, the armature current, then alternate with the same frequency. In an AC motor, the armature winding usually must be linked by an alternating flux and carry an armature current of the same frequency as that of the flux. For an AC machine to work as a synchronous generator, the magnetic flux linking the armature winding usually must alternate with the frequency proportional to the rotor speed. While a three-phase synchronous machine can work as both a generator and a motor, a single-phase machine can only work as a generator.
The progress in technology of high-energy rare-earth permanent-magnets (PMs) has spawned a large number of types of PM electrical machines, in which the required magnetic flux is produced by one or more PMs. In such machines, the PMs can either replace electromagnets in traditional designs, or novel topologies can be developed to make the best use of the properties and characteristics of PMs.
PM machines generally have a multiple stationary pieces (stators) and rotating pieces (rotors). The rotors usually contain many permanent magnets arranged such that their poles alternate in polarity. Each stator typically has windings that are arranged into two sets, and a distributor cycles current to the sets so that each set reverses polarity each successive time it is charged. Such construction permits the use of both poles of the magnets to provide driving forces.
PM machines typically have high full-load efficiency, high output power per volume, high starting torque (in motors), and their maintenance is limited to periodic lubrication of the bearings. The main drawback of PM machines is their incapacity of under-excitation, which may reduce the low-load efficiency. Also, the price of PMs is still relatively high.
Newer PM machines, whose structures are different from conventional PM machines, have been proposed. Some PM machines have transverse flux (TF) paths, meaning that flux paths are parallel to the rotor axis and not perpendicular to the rotor axis as in classic synchronous machines. A TF PM design can provide a substantial increase in the number of magnetic poles, substantially increasing the frequency-to-speed ratio. Consequently, TF motors often can run slowly, with a high torque, while a TF generator often can be driven by a slow prime mover, such as a wind turbine, still producing high output frequency. High frequencies of wind-turbine generators connected to the grid through a matching transformer are necessary for minimization of the transformer size. In the case of both TF motors and generators, a gearbox may not be required. TF machines are currently used in industry as well as in various vehicles, aircraft, and military equipment.
Commutated-flux generators (CFGs) have also been proposed in recent years. A CFG typically has a single permanent-magnet on a stator, a single coil on the stator, an AC flux linkage of a stationary coil, axial flux paths, non-reversible flux in the stator, an outer rotor, and stator/rotor interaction that is typical for variable-reluctance machines. A CFG usually can be classified as an outer-rotor, single-coil, permanent-magnet, variable-reluctance generator, with an operating principle similar to that of flux-reversal machines. In this design, the permanent-magnet is stationary, and the AC flux linkage of the coil is obtained by switching the flux paths instead of rotating a PM rotor.
CFGs typically have a high and readily-varied number of magnetic poles and thus provide a high frequency-to-speed ratio. CFGs typically provide a single coil with a low number of turns, low copper losses as a result of a thick coil wire, low iron losses from flux reversal in the switches only, and effective air-cooling properties because their switches may double as fan blades. CFGs typically have, however, a high potential for excessive flux leakage and fringing as well as use of large air gaps between the pole rings to reduce leakage between them. These gaps also decrease the power density.
A first version of a CFG (CFG-1) has a single permanent-magnet employed as a source of the magnetic field. Numerous flux conductors distribute the magnetic flux. Half of the conductors are in contact with the north pole of the magnet, so that they have positive polarity, and half of the flux conductors are in contact with the south pole, so that they have negative polarity. A number of flux switches are distributed around the periphery of a tubular rotor housing the stator. The switches are sigmoid-shaped flat pieces of steel, which make and break contacts between the flux conductors to alternately open and close a magnetic circuit for the flux. The reversal of polarity of the magnetic flux induces an alternating EMF in the stator coil placed in a channel cut in the flux conductors. Tens of flux conductors can be accommodated on the stator, so even a low rotor speed results in a high frequency and a high voltage EMF in the coil. The CFG-1 design has a prohibitive amount of leakage flux. When the stator is assembled, the north-pole conductors are interspersed with south-pole conductors and their relatively large area and small gap provide low-reluctance leakage paths for the magnetic flux. As a result, only a small amount of flux remains to link the stator coil.
A second version of a CFG (CFG-2) has a structure different than the arrangement of interspersed north and south pole flux conductors of CFG-1. The major components of a CFG-2 include a rotor housing, an assembly of flux switches, a circular permanent-magnet with radial MMF orientation (e.g., north pole outside and south pole inside), a north laminate (e.g., an assembly of north pole tooth-like flux conductors, a stator coil, and a south laminate. Like the CFG-1 design, however, the CFG-2 design has many issues. For example, the power generated is low and the amount of leakage, most of which takes place between unaligned poles, is large.
Another problem with the CFG-2 design arises from the large gaps for reduction of leakage flux. As a result of these gaps, the generator is “loosely packed,”creating a significant amount of unused free space inside the generator. In addition, because the armature coil occupies a small fraction of the machine volume, most of which is filled with iron in order to redirect flux, the CFG-2 provides reduced power density.
Yet another issue with CFG machines is their high cogging torque, which is typical of permanent-magnet single-phase machines generally. The cogging torque is often particularly high in serrated-pole designs with a small but expansive air gap between the stator and rotor. For example, a strong wind is typically required to start a CFG-based wind turbine, rendering operation unreliable at best.
With regard to three phase CFG machines, a three-phase CFG generator can be assembled as a stack of three single-phase generators. In contrast with the classic synchronous machines having periphery-distributed poles and windings, however, the axial arrangement of these components in a three-phase CFG would make it excessively long and bulky.
As noted above, flux leakage can be a significant problem for rotating electrical machines. Flux leakage is flow of flux out of a magnetic material, such as the wall of a pipe, into a medium with lower permeability, such as gas or air.
Curbing the phenomena of flux leakage and armature reaction has been particularly difficult in the design of PM synchronous AC machines. In addition, because ideal conductors or insulators do not exist in any practical sense, only a fraction of the total flux generated in the PMs effectively links the armature winding, while the rest of the flux leaks elsewhere closing through other parts of the machine. The armature reaction consists in the armature current producing its own magnetomotive force (MMF), which opposes the flux coming from the PMs. As a result of both the leakage and armature reaction, the armature voltage decreases with the increase in the current drawn from a generator. A motor with strong armature reaction and high leakage requires higher voltage to produce a given current and torque than that with weak armature reaction and low leakage.
As also discussed above, the need for renewable energy is increasing as traditional fuels become depleted and expensive. Environmental concerns also play an important role in stimulating research on renewable energy sources (e.g., wind turbines). In many applications (e.g., wind-turbine systems or vehicular drives), a high frequency-to-speed ratio is required to allow direct (e.g., gearless) mechanical connection between the generator and prime mover or the motor and load.